1. Field of the Invention
The invention relates to image capture apparatus such as a video camera for producing video image signals and to a programmable filter and to an image sensor suitable for use with such an image capture apparatus.
2. Description of the Prior Art
Conventional television broadcasting systems are designed to process images containing an exposure range of the order of four to five iris stops. The image sensor of most television cameras for capturing images for use in television broadcasting systems are, in fact, capable of sensing images having a greater exposure range than four to five iris stops. However, because of the limitations of the broadcast systems, any light entering the image sensor outside this range, although detected, will not be recorded or transmitted.
It has been suggested to apply compression curve techniques, similar to those used on film, to video television images. The compression curve techniques apply a scaling factor to image intensities above a certain level in order to produce signals lying within an exposure range which can be accommodated in television broadcast systems. However, the use of compression curves does not provide an acceptable solution in many cases. For example, many images comprise ranges of intensity values separated by a step rather than a single homogenous range of intensity values. An example of this would be the classical back lighting situation where the foreground of an image lies within a room and the background lies outside of the room viewed, for example, through a window. Situations of this type can be accommodated by human intervention, for example by a cameraman putting in a selective or gradient type filter or by means of double exposure and keying techniques in post production. These techniques are applicable as long as the scene is static. However, the problems of accommodating such scenes are particularly acute where the camera is panned.
An international patent application with the publication number WO 90/01844 describes a video imaging apparatus having a multiplicity of photo sensitive detectors arranged in an array and adaptive control circuitry for determining integration times for the photosensitive detectors so that they operate in different operating ranges to provide enhanced dynamic ranges for the detectors. The signal values output by the detectors are then multiplied by factors inversely proportional to the integration times to produce an enhanced linear range of intensity values within the image with the relationship between the original intensity values of the viewed scene being maintained across the image. Conventional compression techniques are then applied to adapt this enhanced linear range of intensity values to one suitable for output from the apparatus to television equipment connected thereto. Another international patent application with the publication number WO 90/01845 describes an apparatus which comprises a plurality of cameras for producing a plurality of video images of a scene at different exposures levels and apparatus for processing the plurality of video images to produce a combined video image including image information from the plurality of video images to include enhanced detail at local areas therein. Both of these international patent applications deal with enhancing the dynamic range which can be sensed by the sensed elements of an image capture apparatus. In both cases the aim is to maintain the relationship between the original intensity values of the scene being viewed across the image. They employ conventional compression techniques to adapt the enhanced range to one suitable for output. However, they do not address the problem of producing an output video signal from an image capture apparatus where the incident light on the image sensor of the image capture apparatus is within the capability of that image sensor, but exceeds the exposure range which can be accommodated by television equipment connected to receive the output from the image capture apparatus.
An object of the invention is to provide an image capture apparatus such as a video camera for producing video image signals for a television system, which apparatus is able automatically to accommodate significant differences in image intensities.